The present invention relates to a method for heat treating a synthetic quartz glass body for optical use. Such quartz glass bodies are highly homogeneous, having favorable high optical transmittance and high resistance against laser radiation, and they are suitable for use as an optical member in lithographic apparatuses working with an excimer laser. The present invention also relates to a heat treatment method for heat treating the synthetic quartz glass body and it relates to a heat treatment apparatus used for this method.
Photolithography is a technology which comprises transferring the pattern provided on a photomask onto a wafer by using laser radiation. It is economically superior to other techniques using electron beams and X rays. Hence, it has been widely used heretofore in the steppers for producing semiconductor integrated circuits.
Recently, with an increase in the production of xe2x80x9cLarge Scale Integrated Circuitsxe2x80x9d (LSIs) with finer patterns and an increase in the degree of integration, light with still shorter wavelength is demanded for the exposure light source. Steppers using i-lines (365 nm in wavelength) capable of fabricating patterns with line widths in a range of from 0.4 to 0.5 xcexcm or those using KrF excimer lasers (248.3 nm in wavelength) capable of patterning lines having a line width of from 0.25 to 0.35 xcexcm have been practically used heretofore, and more recently, there is being developed a stepper using an ArF excimer laser (emitting light 193.4 nm in wavelength) for practical use, which is capable of patterning lines 0.13 to 0.2 xcexcm in line width. Thus, it is demanded for the optical members for use in the apparatuses using ArF excimer laser lithography to satisfy requirements such as homogeneity, transmittance, resistance against laser radiation, etc., at high levels heretofore never realized.
As a material capable of satisfying such demands, high purity synthetic quartz glass is being used, and the optical transmittance and the resistance against laser radiations of such materials have been improved by optimizing the production conditions. At the same time, optical properties such as the homogeneity and birefringence are further increased. Among them, to improve the homogeneity and to reduce the birefringence, it is required to perform a heat treatment inclusive of gradual cooling (annealing treatment) during the production process of the optical member to remove the strain of the quartz glass. As a method of the heat treatment, generally employed was a method comprising maintaining the glass at high temperatures inside the heating furnace.
The hydrogen molecules dissolved inside a quartz glass contributes to the resistance against short wavelength radiations, and particularly, to the resistance against laser radiations. For instance, it is known that a drop in transmittance or a generation of fluorescent light, or an increase in refractive index of a quartz glass containing hydrogen molecules at an amount not lower than a certain level can be suppressed by exposure with an excimer laser for a long period (see JP-A-Hei3-88743 and JP-A-Hei9-124337)
However, in the heat treatment described above, because quartz glass is exposed to high temperatures for a long time, there occurs a problem that the quantity of hydrogen molecules incorporated inside the quartz glass is lowered, thereby resulting in a decrease in resistance against laser radiations.
Furthermore, since the heat treatment above is commonly performed by holding the quartz glass inside a heating furnace (e.g., an electric furnace) for a long duration of time, there was found another problem of process contamination, because the quartz glass under treatment would catch up impurities such as the alkaline metal elements discharged or transferred from the furnace material of the heating furnace or the jigs, or from the atmosphere, etc. Since the metallic impurities incorporated in the quartz glass for optical use induce a drop in the transmittance and the resistance against laser radiations, the contamination due to metallic impurities, particularly Na, must be suppressed as low as possible to obtain an optical member suitable for use in laser lithographic apparatuses.
In order to overcome those problems in the heat treatment process above, there is proposed a method comprising performing the heat treatment by covering the object to be treated with a proper cover. In JP-A-Hei10-279322 it is proposed to cover the object to be treated with synthetic quartz glass to prevent the object to be treated from being contaminated. However, prevention of contamination is the only object of the invention, and no consideration is made to the lowering of the concentration of dissolved hydrogen molecules. In JP-A-Hei8-91857 is proposed a method for obtaining a highly homogeneous quartz glass, which comprises thermally treating the object to be treated by covering the object with a sheet or a powder of silica. However, similar to the case of JP-A-Hei10-279322, in this invention again, no consideration is made on the measures for lowering the concentration of dissolved hydrogen molecules. Furthermore, although there is known doping of hydrogen molecules as a method of recovering the lowered concentration of hydrogen molecules, this method requires a special step for doping, and this led to a longer time of production and to an increase in production cost.
The present invention has been achieved in the light of the aforementioned problems, and an object of the present invention is to provide a synthetic quartz glass body for optical use showing improved transmitting properties and resistance against laser radiation, a method for heat treating the same, and a heat treatment apparatus for performing the method.
In general, the present invention provides for the heat treatment of an optical quartz glass object, wherein the heat treatment is conducted within an enclosed space with a SiO2 powder surrounding the object. The SiO2 powder has a relatively high surface area, and therefore tends to absorb impurities, such as alkaline metals, before they reach the quartz glass object. An enclosed vessel is preferably provided around the SiO2 and optical quartz glass object during treatment, to contain the hydrogen and prevent external contamination. The SiO2 powder has a relatively high initial hydrogen molecule concentration, e.g., a mean concentration of at least about 1019 molecules/cm3, which is higher than the desired end hydrogen concentration of the optical quartz glass object, e.g., 2xc3x971017 molecules/cm3 or higher, and therefore the hydrogen concentration in the environment around the optical quartz glass object tends to impede loss of hydrogen from the optical quartz glass object during the heat treatment process.
The problems above are solved by providing any of the constitutions of the present invention as follows:
(1) A method for heat treating a synthetic quartz glass body for optical use, comprising heating the quartz glass body in a heating furnace while said quartz glass body is covered with an SiO2 powder, wherein the SiO2 powder have a mean dissolved hydrogen molecule concentration of 1xc3x971019 molecules/cm3 or higher.
(2) A method as stated in (1) above, wherein the total weight of said SiO2 powder accounts for 25% or more of the weight of the synthetic quartz glass body.
(3) A method as stated in (1) or (2) above, wherein said SiO2 powder is a synthetic SiO2 powder containing 30 wt-ppb or less of Na.
(4) A method as stated above in one of (1) to (3), wherein at least 95% of the weight of said SiO2 powder consists of particles having a diameter of 1,000 xcexcm or less.
(5) A synthetic quartz glass for optical use that is produced by the heat treating method as stated above in one of (1) to (3), said synthetic quartz glass for optical use having a dissolved hydrogen molecule concentration of 2xc3x971017 molecules/cm3 or higher and an initial transmittance for a light 193.4 nm in wavelength of 99.7% or higher.
(6) A heat treatment apparatus for heat treating of a synthetic quartz glass body for optical use, comprising a quartz glass vessel covered by a lid and having a volume of 1.5 times or more of the volume of the synthetic quartz glass body to be enclosed therein, said vessel is placed inside an heating furnace and filled with an SiO2 powder having a mean dissolved hydrogen molecule concentration of 1xc3x971019 molecules/cm3 or higher covering the surface of the synthetic quartz glass body.
(7) A heat treatment apparatus as stated in (6) above, wherein the total weight of said SiO2 powder accounts for 25% or more of the weight of the synthetic quartz glass body.
(8) A heat treatment apparatus as stated above in one of (6) to (7), wherein said SiO2 powder is a synthetic SiO2 powder containing 30 wt-ppb or less of Na.
(9) A heat treatment apparatus as stated above in one of (6) to (8), wherein at least 95% of the weight of said SiO2 powder consists of particles having a diameter of 1,000 xcexcm or less.